I don't know what your mother tongue is but, as you wrote your question in English, I guess you master this language. And in English the tenses used in a similar situation would be the same, i.e. "I saw some sheep cross the road" because I observed the process, and "The police spotted/saw him entering the doctor's surgery" because they saw him while he was performing that action.
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PaolaApr 22 '13 at 0:44

1 Answer
1

In the first sentence, the important observation is that the sheep crossed the road. What was seen is the crossing. The fact that I've seen sheep is (mostly) irrelevant.

In the second sentence, the relevant is that the police spotted him. What was seen is him. The last part of the sentence merely specifies the conditions, but they are considered less relevant. It could be rephrased like this: